This invention relates to oil and gas drilling, and more particularly to apparatus and methods for gathering seismic data relating to subsurface formations.
In the oil and gas exploration industry, it is useful for geologists to understand underground conditions, such as the type of source rock, reservoir rock, or entrapment areas that may indicate the presence of petroleum and other hydrocarbon reserves. Geologists have historically used several techniques to accomplish this end. For example, geologists may observe surface features, such as rock and soil types, or take core samples. Satellite images, gravity meters, magnetometers, sniffers, and the like, have also been used to detect conditions that may indicate the presence of oil, gas, or other hydrocarbons.
Currently, seismology is perhaps the most commonly used technique to detect the presence of oil and gas. Seismology is the science of mapping and detecting subsurface structures by interpreting the behavior of shock waves traveling though subsurface rock, water, hydrocarbons, or other materials. Shock waves may be generated by devices such as compressed-air guns, thumper trucks, and explosives. These shocks waves may be detected and measured by hydrophones, geophones, or seismometers after the shock waves pass through or are reflected by subsurface structures. These measurements may be used to create seismic surveys. Also, with the advent of three-dimensional seismic surveys, drillers can tap into hydrocarbon-bearing reservoirs with greater accuracy than previous two-dimensional surveys. Nevertheless, although seismology generally provides more usable information than other detection techniques, a drilling company may still only have a 10% chance of finding a new oil field using the technique.
Although above-surface seismic exploration methods have been used extensively, more recently, seismic measurements are taken downhole inside a well or borehole. This method may improve the quality of seismic data gathered because the measurements are taken closer to subsurface formations of interest, and because seismic wave attenuation and noise are reduced. Such methods are often referred to as downhole seismic, and more particularly as single-well seismic and cross-well seismic. These measurements are generally taken be placing seismic receivers and seismic wave generators in the borehole.
Nevertheless, current downhole seismic data gathering techniques are hindered by several shortcomings. For example, because current seismic devices, such as geophones and seismic wave generators, are often lowered into the borehole on wirelines, it may be necessary to temporarily stop drilling while the wireline is lowered into or retrieved from the borehole. This process can be quite expensive, not only because of the rig time consumed, but because running the seismic logs can be very expensive.
Thus, what are needed are improved apparatus and methods for gathering seismic data at high speeds, while minimizing drill rig down time. What are further needed are apparatus and methods for gathering seismic data while drilling or after drilling has stopped but without lowering a wireline into the borehole. Finally, what are needed are apparatus and methods for transmitting large quantities of seismic data to the surface at high speeds.